An amorphous oxide film composed of indium oxide, zinc oxide and gallium oxide has attracted attention as a transparent conductive film or a semiconductor film (used in a thin film transistor or the like) since it has transmittance to visible rays and a wide range of electric properties from a semiconductor to an insulator.
The oxide film is formed by a physical film-forming method such as sputtering, pulse laser deposition (PLD) and deposition or a chemical film forming method such as a sol-gel method. A physical film-forming method such as sputtering has been mainly studied since a film is uniformly formed in a large area at a relatively low temperature.
When forming an oxide thin film by a physical film-forming method such as sputtering, it is common to use a target formed of an oxide sintered body in order to form a uniform film stably and efficiently (at a high film-forming speed).
As the representative oxide films (conductive film/semiconductor film), an oxide film composed of indium oxide, zinc oxide and gallium oxide can be given, for examples. As the target (mainly, a sputtering target) for forming these oxide films (normally, amorphous films), studies have been made mainly on a crystal form with a homologous structure such as InGaZnO4 and In2Ga2ZnO7.
For example, Patent Documents 1 to 4 each disclose a target having a homologous structure represented by InGaZnO4(InGaO3(ZnO)). However, since the homologous structure hardly causes oxygen deficiency, it was necessary to allow oxygen deficiency to cause to reduce the resistance by conducting a reduction treatment at a high temperature (Patent Document 1). Therefore, a production method which does not generate a highly insulative Ga2O3 crystal phase (Patent Document 3), a method in which a positive tetravalent metal is added or the like (Patent Document 4) and a target formed of a mixture of a hexagonal compound represented by InGaZnO4 and a compound with a spinel structure represented by ZnGa2O4 (Patent Document 4) have been studied. However, they have problems that the advantageous effects are limited and management is difficult due to an increased number of constituting elements.
Studies have been made on the production of various thin film transistors by changing the composition ratio of indium oxide, zinc oxide and gallium oxide (Patent Document 5). However, since the studies on the target at each composition were not sufficiently made, the specific resistance of these thin film transistors was still high. In addition, an example was disclosed in which an amorphous oxide semiconductor film and a thin film transistor were formed by using an In—Ga—Zn—O sintered body with a metal composition ratio In:Ga:Zn=30:15:55 (Patent Document 6). However, there was a problem that the Ga content of a thin film was significantly decreased such that it became about two-third of the Ga content of the target. This suggests improper properties of the target, however, no studies were made on the target properties and the improvement thereof.